Apple Mail to PC image received as icon

Using Apple Mail and inserting cut slide from Powerpoint/Keynote. This appears as an image in the content of email. According to clients who have received email, they are receiving as an attachment instead of the in content image.
For marketing purposes having the image show in content is important for us.

Can anyone offer any suggestions to ensure that emails are received by PC users have images in content? Is this related to their own Mail system setup?

NB: from Mac to Mac this has not been a problem and View in Place and View as Icon has worked fine.